Chassis/body marriage machine with helical push actuator

ABSTRACT

A machine for supporting and lifting an automotive chassis module into position for assembly with an automobile body includes a mobile vehicle having at least one fixture arranged above the vehicle for receiving and supporting the chassis module and a guided lift mechanism for moving the fixture vertically between lowered and raised positions. The lift mechanism includes a helical push actuator having a drive motor and at least one band that can be advanced and retracted in a vertical helix with an end of the band being coupled to the fixture to move the fixture between the lowered and raised positions by helical advancement of the band under control of the motor. The lift mechanism includes spaced support members that each extend between the fixture and the vehicle to guide the fixture in the vertical direction and restrain the fixture from moving horizontally.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority of U.S. ProvisionalApplication No. 60/384,474, filed May 31, 2002, the entire contents ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

[0002] This invention relates generally to lift machines used in theautomotive vehicle manufacturing industry for marrying chassis moduleswith vehicle bodies and, more particularly, for lifting the chassismodules into place underneath suspended vehicle bodies for subsequentfastening of the chassis modules to the vehicle bodies.

BACKGROUND OF THE INVENTION

[0003] In the manufacturing of automotive vehicles, it is customary topreassemble components of the chassis of the vehicle before marryingthem with the vehicle body. The bringing together of the chassis andbody takes place on a continuously moving conveyor line. The body istypically conveyed overhead by a conveyor, and the chassis components tobe married to the body are supported by a fixture of a moving liftmachine that operates to move the chassis components into positionbeneath the moving body while lifting the chassis components intoposition for assembly with the body.

[0004] Many of the lift machines in present use employ a pneumaticallypowered hydraulic lift mechanism to raise and lower the fixture orfixtures that carry the chassis components. Some machines have a singlefixture, whereas others are equipped with two such fixtures provided atopposite ends of the lift machine. Each fixture has its own hydraulicsystem and the two are often synchronized to operate in unison.

[0005] The hydraulic systems each include a fluid cylinder that ismounted on the machine and secured to its associated fixture. A pair ofmulti-stage telescoping vertical guides are provided on opposite sidesof each cylinder and guide the fixture vertically. A longitudinallyextending cabinet projects above a base of the machine in the spacebetween the fixtures and houses various components associated with thecontrol and operation of the hydraulic/pneumatic systems. Among thecomponents accommodated within the cabinet is a reservoir containinghydraulic fluid to be supplied to the cylinders, and an onboardpneumatic pump system that drives the hydraulics. Then pump system hasonboard rechargeable air tanks also housed in the cabinet that hold asupply of pressurized air for operating the pump. The pump in turncontrols the flow of hydraulic fluid to and from the cylinders in orderto raise and lower the platforms.

[0006] While the traditional hydraulic/pneumatic lift mechanism of suchmachines performs satisfactorily in raising and lowering the platforms,it has several inherent disadvantages which add cost and complexity tothe operation, many of which are unique to the art of chassis componentlift machines.

[0007] The hydraulic systems have various seals, fittings and othercomponents that are prone to frequent leakage and failure and requirethe constant attention of skilled workmen to maintain them in workingorder. It also requires that a fairly large capacity hydraulic fluidreservoir be carried on board the machine to supply fluid to thecylinders, adding to the size, cost and weight of the machines.

[0008] The rechargeable pneumatic pump system presents its own set ofproblems. The pump relies for its power on the supply of pressurized airfrom the onboard tanks. When the air in the tanks is exhausted, the pumpand thus the hydraulics are no longer operational. A spent machine mustbe routed out of the normal assembly loop to a specialized rechargestation where the tanks are refilled with pressurized air. During thistime, another machine must be introduced in its place to support thecontinuous assembly operation. Since a full charge typically provides atmost three lift cycles before requiring recharging, a number of extramachines must be kept on hand to support the assembly operation, addingfurther to the cost of supporting the operation. The specially equippedcharging stations also add to the cost of the operation and can occupyvaluable floor space of a facility.

[0009] Another objection inherent with the existing hydraulic/pneumaticlift system is that the elevated cabinet needs to be fairly tall inorder to house the various components of the hydraulic/pneumatic system.The presence of the elevated cabinet can obstruct full access to thecomponents supported on the fixtures to those assisting in the assemblyoperation, and particularly their step-through passage across themachine, making it less convenient to reach certain areas.

[0010] One solution proposed to overcome problems inherent inhydraulic-based systems is the use of a rigid chain used to providevertical lift in place of the hydraulic lift mechanism. Such a system isthe subject of U.S. Pat. No. 6,109,424, which is assigned to theassignee of this invention.

[0011] A chassis lift apparatus constructed in accordance with thepresent invention provides yet another alternative to conventionalhydraulic/pneumatic chassis lift machines.

SUMMARY OF THE INVENTION

[0012] In accordance with one aspect of the present invention, there isprovided an apparatus for lifting and supporting an automotive chassismodule in position to be assembled with an automotive body along amoving assembly line. The apparatus comprises a mobile vehicle, at leastone fixture arranged above the vehicle for receiving and supporting thechassis module, and a guided lift mechanism for moving the fixturevertically between a lowered position and a raised position. The liftmechanism includes a push actuator having a drive motor and at least oneband that can be advanced and retracted in a vertical helix with an endof the band being coupled to the fixture to move it between the loweredand raised positions by helical advancement of the band under control ofthe motor. The lift mechanism further includes at least one supportmember extending between the fixture and vehicle operative to guide thefixture in the vertical direction and restrain the fixture from movinghorizontally.

[0013] In accordance with another aspect of the present invention, thereis provided a method of supporting and lifting an automotive chassismodule for assembly with an automotive body. The method comprising thesteps of:

[0014] loading the chassis module onto a fixture supported at a loweredposition and coupled to a push actuator mounted on a mobile vehicle;

[0015] operating the push actuator to raise the fixture vertically byupward advancement of at least one helical band; and

[0016] providing at least one support member between the fixture and thevehicle to guide the fixture during its vertical movement whilerestraining the fixture from horizontal movement.

[0017] The push actuator enables construction of a chassis/body marriagelift machine that can maintain a very low profile when retracted whileadvancing the fixture to a total height of many times its retractedheight. Other features and advantages of the present invention willbecome apparent after reading the following description of theillustrated embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A preferred exemplary embodiment of the invention willhereinafter be described in conjunction with the appended drawings,wherein like designations denote like elements, and wherein:

[0019]FIG. 1 is a diagrammatic elevational view depicting an embodimentof the chassis lift machine of the present invention as it would beutilized as part of an automobile manufacturing production line;

[0020]FIG. 2 is a front view showing the lift mechanism used in thechassis lift machine of FIG. 1;

[0021]FIG. 3 is a plan view of the lift mechanism of FIG. 2;

[0022]FIG. 4 is, a side view of the helical lift mechanism used in theembodiment of FIG. 1;

[0023]FIG. 5 is a plan view of the helical lift mechanism of FIG. 4;

[0024]FIG. 6 is a side sectional view of the lift mechanism of FIG. 2;and

[0025]FIG. 7 is a rear view of the lift mechanism of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] As shown in FIG. 1, a chassis/body marriage lift machine (orchassis lift vehicle or apparatus) of the present invention isdesignated generally at 10 and is shown as it would be used at achassis/body marriage and assembly station 12 of an automotiveproduction line. Automotive vehicle bodies, such as that shown at 14,are brought into station 12 one at a time by an overhead clamshellconveyor 16 that is supported by and moves around an endless overheadrail 18. Automotive chassis modules 20 are also brought into station 12via an overhead conveyor and then are placed onto lift machine 10 forsubsequent assembly into vehicle body 14.

[0027] To provide a continuously operating production line, the liftingand assembly of the chassis module 20 into vehicle body 14 is carriedout while the vehicle body 14 moves along the clamshell conveyor 16.Thus the lift machine 10 runs along a floor track 22 (or is self-guided)underneath the conveyor 16 while chassis module 20 is lifted andfastened into the vehicle body 14. Movement of lift machine 10 alongtrack 22 and the required synchronization of lift machine 10 withconveyor 16 are well known to those skilled in the art and willtherefore not be elaborated upon.

[0028] The machine 10 includes a wheeled vehicle 26 that serves as thebase or framework of the machine 10 on which other components of themachine are supported. There are generally three types or classes ofvehicles 26 that may be utilized in conjunction with the lift mechanismof the invention to be described. They include those that areself-propelled but guided by a floor track such as that shown at 22, aso-called tow-veyor type vehicle which is towed by a floor cable or thelike along a floor track 22, or a self-powered, self-guided typevehicle, known generally as an automatic guided vehicle or AGV, which isself-propelled and programmable to be self-guided without the assistanceof a floor track along a preset path. Of course, other vehicle typescould be used and are contemplated as equivalent provided they aresuitable for the intended purpose of marrying chassis components toautomotive bodies.

[0029]FIG. 1 illustrates a self-powered vehicle 26 having an on-boarddrive motor 27 that drives the vehicle 26 along the guide track 22 inconventional manner. The use of chassis lift machine in assembly lineproduction is known, as exemplified in U.S. Pat. No. 6,109,424, theentire contents of which are hereby incorporated by reference.

[0030] The fastening of chassis module 20 to vehicle body 14 can becarried out either manually or automatically. In the illustratedembodiment shown in FIG. 1, fastening is carried out automatically andin a conventional manner using a mobile screw station 28. Screw station28 moves synchronously with lift machine 10 and vehicle body 14, duringwhich time chassis module 20 is secured to vehicle body 14 usingfasteners (not shown). Automatic fastening is accomplished using apallet 30 that is attached to lift machine 10 and that is used to holdand properly locate chassis module 20 for assembly into vehicle body 14.Pallet 30 can be used to hold each of the required fasteners 32 at theproper location in preparation for fastening of the chassis module tothe vehicle body.

[0031] Pallet 30 also includes nut drivers (not shown) for each of thefasteners.

[0032] Screw station 28 includes motorized drives 34, each of whichmates with a corresponding nut driver in pallet 30 to provide automatedtightening of the fasteners. As screw station 28 moves along track 22with lift machine 10, it extends its motorized drives 34 upwards untilthey engage their associated nut drivers. The fasteners can then beautomatically tightened into vehicle body 14.

[0033] Screw station 28 must fasten chassis module 20 to vehicle body 14while the chassis module is being held in an upper position against theunderside of the vehicle body. Thus, lift machine 10 must be able tohold the chassis module in the upper position without blocking access toany of the fastening points. In most vehicle designs, there is anapproximately six inch wide longitudinal strip along the length of thevehicle in which no fasteners 32 are needed for fastening of the chassismodule to the vehicle body. This strip defines an interference-freeregion 36 that extends lengthwise along the longitudinal axis of thelift machine and widthwise between opposed fastening regions 38, 40.

[0034] To avoid interference with screw station 28, lift machine 10includes at least one and often two guided lift mechanisms 42 mounted onthe vehicle 26 at opposite front and rear ends there. The liftmechanisms are designed to extend only within this interference-freeregion during the raising and lowering of chassis module 20.

[0035] Although the lift machine depicted in FIG. 1 is shown front andrear guided lift mechanisms 42, it will be understood that only a frontor rear lift mechanism may be required, depending upon the application.In this regard, it should be noted that chassis module 20 can be thefull vehicle chassis module, including the engine, transmission, drivetrain, and suspension requiring both the front and rear lift mechanisms42, or can be a portion of the full vehicle chassis module, such as afront or rear chassis module, in which case only one of the front andrear lift mechanisms 42 may be required.

[0036] In the description that follows, reference will be made to asingle lift mechanism 42. However, for those applications utilizing bothfront and rear lift mechanisms, the other lift mechanism could be andpreferably is identical to the one described below and the two liftmechanisms could be run synchronously to raise a full chassis moduleinto place. For this purpose, the two lift motors used to raise thechassis module could include position encoders or other position sensorsto permit the synchronous operation.

[0037] As shown in FIGS. 2, 3, 6 and 7, lift mechanism 42 is constructedpreferably as a separate, self-contained unit apart from the vehicle 26and includes push actuator base 44, support members 48 and 49, fixture52, an upper base 53, and a push actuator 64. The motor assembly 54 isrigidly mounted on the push actuator. Also included is a mounting plate56 that is mounted on the vehicle 26, which is spaced a predeterminedfixed distance above the shop floor 22. It is preferred that the liftmechanism 42 be mounted releasably on vehicle 26, so that the mechanismcan be attached and detached from the vehicle 26 when needed formaintenance or replacement. Accordingly, bolts 58 or other suitablefastening means may be employed to secure the mounting plate 56 to thevehicle 26. Alternatively, the mounting plate 56 could be permanentlyaffixed to the vehicle 26 such as by weldments or the like.

[0038] Support members 48, 49 operate to guide the upper base 53 in thevertical direction while restraining the upper base 53, and thus thefixture 52, against horizontal movement relative to the base 44 of thelift mechanism 42. Such support of the fixture 52 helps maintain thestability of the lift mechanism 42 and prevents undue lateral forcesfrom being exerted on the lift mechanism 42. Support members 48, 49preferably comprise vertical telescoping guides or multistage guides, asillustrated, of the type constructed from telescoping cylindrical columnmembers that extend freely in the vertical longitudinal direction withthe movement of the upper base 53, but restrain the upper base 53against horizontal movement. The support members are housed inappropriately sized hollow structures 88, 89 that allow for retentionand storage of the support members 48, 49. There may be includedappropriate positional control mechanisms such as the switchingmechanism 87 shown in FIG. 2, to control the position of the supportmembers 48, 49. In addition to or in lieu of the telescoping guides, anyother suitable type of support member can be used that guides thefixture 52 vertically during the lift while restricting it fromhorizontal movement; for example, a scissors linkage can be used.

[0039] The fixture 52 is attached to upper base 53 by a conventionalfloating plate mechanism 66 that is slidably supported on thehorizontally restrained upper base plate 53. The floating platemechanism 66 is movable horizontally relative to the upper base plate 53both longitudinally and laterally of the vehicle 26 to enablecorresponding adjustment in the position of the fixture 52 and thus thepallet 30 and its chassis module 20.

[0040] The push actuator 64 can be identical to that described in U.S.Pat. No. 4,875,660, the entire contents of which are hereby incorporatedby reference.

[0041] Alternatively, the modified push actuator disclosed in U.S. Pat.No. 6,547,216 can be used for push actuator 64, and the entire contentsof that patent are also hereby incorporated by reference. Push actuator64 can be, for example, a model HD9 Spiralift™, available from PacoCorp. of Quebec, Canada. Referring now to FIG. 4, salient features ofthe push actuator 64 will now be described, and reference is made to theaforementioned U.S. Pat. Nos. 4,875,660 and 6,547,216 for furtherdetails concerning the construction and use of various implementationsof push actuator 64.

[0042] As shown, the push actuator 64 has an annular base 116 anchored,by means of L-shaped ground anchors 118 and nuts 120, or any othersuitable anchoring means.

[0043] A rotor 122, in the form of a cylindrical body with a lowerradially-outwardly-extending flange 124, is rotatable on the base 116 bymeans of roller bearings 126 engaging annular V-shaped grooves made inthe base 116 and lower flange 124. Thus, the rotor 122 is rotatableabout the central axis of the base 116 and rotor 122. The peripheraledge of the lower flange 124 forms gear teeth 128 all around, in meshingengagement with a driving gear 130 driven in rotation in both directionsby a suitable motor 54.

[0044] A first continuous annular band 132, which will hereinafter becalled the horizontal band for the particular application shown, iswound in an helix and is of such an external diameter as to fit withinthe rotor 122 and the base 116. The turns of this horizontal annularband can be successively lifted to provide a space therebetween. Thelower end of the horizontal band 132 rests on the floor or ground G,while its upper leading end 134 is secured underneath the platform 114.

[0045] A second band 136, hereinafter called the vertical band for theparticular application described, is wound in spiral form, the bandbeing continuous, and when in stacked condition, located within anannular storage box 138, disposed on the outside of rotor 122 androtatably supported with respect to the rotor by means of threeequally-angularly spaced L-shaped brackets 140 fixed to the outside ofrotor 122 by bolts 142 and provided at their outer end with an idleroller 144 rotatably supporting the storage box 138.

[0046] As shown in FIG. 4, the coil formed by vertical band 136 has aminimum diameter which is greater than the external diameter of thehorizontal band 132, so as to clear the horizontal band when the latteris fully stacked, or when it is being extended. The radially outer endof the vertical band 136 need not be secured to the storage box 138. Theradially inner leading end of the vertical band 136 extends through arectangular opening 146 of rotor 122, inward of the rotor 122, and istapered widthwise, to be secured to the underside of the platform 140 bysuitable means, such as a cross-sectionally I-shaped member 150. Leadingend 148 fits between the radially inner flanges of member 110, such thatthe member 110 is similar to longitudinally tapered leading end 148, themember 110 being directly secured underneath the platform 114.Therefore, the platform is maintained in level condition despite thefact that the vertical band 136 extends upwardly along a helix.

[0047] The rotor 122 is provided along its internal surface with aseries of equally-spaced idle rollers 152, each mounted on a horizontalshaft 154, secured to the rotor by means of a nut 156. The idle rollers152 are generally equally spaced for a full circle around the inside ofthe rotor 122 and arranged in a helix in accordance with the desiredpitch of the assembly of the horizontal band 132 and vertical band 136.As shown in FIG. 4, the leading roller 152A is longitudinally alignedwith the elongated opening 146, itself inclined in accordance with theabove-noted pitch. The trailing roller 152B is vertically above andspaced from the leading roller 152A.

[0048] It will be understood that when the rotor 122 is rotated in adirection to elevate platform 114, the idle rollers 152 will roll underthe radially outer margin of the turns of the horizontal band 132, so asto space the same from the underlying adjacent turn. At the same time,the vertical band 136 enters opening 146 and is guided radially inwardlyby the inner flat faces of the rollers 152 to be inserted between twoadjacent turns of the horizontal band 132. As the horizontal band 132 isbeing elevated by rotation of the rotor 122, and consequently of therollers 152, along an helix concentric with the central axis of therotor 122, the vertical band 136 is gradually inserted between theadjacent turns of the horizontal band 132, so as to gradually build up atube or column. Insertion of the vertical band 136 is facilitated, dueto the weight of the horizontal band 132 at the point of insertion,which horizontal band 132 form a gap 160.

[0049] Location structure is included to properly locate the verticalband 136 between adjacent turns of the horizontal band 132. The locationstructure includes a plurality of studs 162 press-fitted within variousseries of equally-spaced corresponding through-bores 164 made throughthe horizontal band 132. The through-bores 164 are preferably positionedat an equal distance from the outer peripheral edge of the horizontalband 132. A portion of the studs 162 are upwardly directed, while othersare downwardly directed. This upward and downward positioning of thestuds 162 locates both the top and bottom edges of the vertical band136.

[0050] A portion of the through-bores 164 are left without any studs162, so as to receive the studs 162 of the next adjacent turn of thehorizontal band 132 when in a retracted position. The protruding portionof the studs 162 are preferably sized such that they are not longer thanthe thickness of the horizontal band 132.

[0051] In this manner, a flat stacking of the horizontal band 132 isobtained for storage in a minimum of height. As shown in FIG. 5, theassembly of two adjacent turns of the horizontal band 132, with a turnof the vertical band 136 inserted therebetween, forms a substantiallyI-shaped cross-section. The resulting assembly is very strong and cansupport a very large load, and yet it is very stable transversely of thecentral axis. Depending on the length of the two bands 132 and 136, thepush actuator 64 of the invention can have a very long stroke comparedto its length when the two bands are in retracted position.

[0052] Preferably, the vertical band 136 is pre-rolled to possess whenunstressed, a radius of curvature about equal to that of the assembledtube. This facilitates proper insertion of the vertical band 136 betweenthe turns of the horizontal band 132.

[0053] Magazine or box 138 is free to rotate about the tube axis topermit vertical band 136 insertion and removal despite the resultingvariation of the internal diameter of the coil formed by the verticalband 136 in the box 138.

[0054] In constructing and operating a chassis module lift machine 10,at least one and in many applications two mechanical push lift devices42 are constructed in the above described manner and are mounted onopposite ends of a mobile lift machine vehicle 11. The motors 54 ofthese lift mechanisms are utilized to lower the fixtures 52 to thelowered positions, retracting and storing the support guides 48, 49 andthe bands of the helical push actuators 64 as previously describedabove. The machine is moved into position beneath a vehicle body and themotors 54 are operated to extend the push actuator 64 to lift thefixture 52 and thus the chassis modules 20 to the raised position. Atleast one and preferably two vertical guides 48, 49 are provided betweenthe vehicle and each upper base 53 to support the fixtures 52 duringtheir vertical movement.

[0055] It will thus be apparent that there has been provided inaccordance with the present invention a chassis/body marriage machineand method which achieves the aims and advantages specified herein. Itwill of course be understood that the foregoing description is of apreferred exemplary embodiment of the invention and that the inventionis not limited to the specific embodiment shown. Various changes andmodifications will become apparent to those skilled in the art and allsuch variations and modifications are intended to come within the scopeof the appended claims.

[0056] As used in this specification and appended claims, the terms “forexample,” “for instance,” and “such as,” and the verbs “comprising,”“having,” “including,” and their other verb forms, when used inconjunction with a listing of one or more components or other items, areeach to be construed as open-ended, meaning that the listing is not tobe considered as excluding other, additional components or items. Otherterms are to be construed using their broadest reasonable meaning unlessthey are used in a context that necessarily requires a differentinterpretation.

1. Apparatus for lifting and supporting an automotive chassis module inposition to be assembled with an automotive body along a moving assemblyline, said apparatus comprising: a mobile vehicle; at least one fixturearranged above said vehicle for receiving and supporting the chassismodule; and a guided lift mechanism for moving said fixture verticallybetween a lowered position and a raised position, said mechanismincluding a push actuator having a drive motor and at least one bandthat can be advanced and retracted in a vertical helix with an end ofsaid band being coupled to said fixture to move said fixture betweensaid lowered and said raised positions by helical advancement of saidband under control of said motor, said lift mechanism further includingat least one support member extending between said fixture and saidvehicle operative to guide said fixture in the vertical direction andrestrain said fixture from moving horizontally.
 2. The apparatus ofclaim 1, wherein said vehicle comprises an automatic guided vehicle. 3.The apparatus of claim 1, wherein said vehicle comprises a tow-veyor. 4.The apparatus of claim 1, including a base removably mounted on saidvehicle, and wherein said guided lift mechanism is mounted on said base.5. The apparatus of claim 1, wherein said support member comprises atleast a pair of vertically extendible guides spaced laterally from andextending parallel to said vertical helix of said push actuator.
 6. Theapparatus of claim 1, wherein said at least one support member comprisesat least three vertically extendible guides spaced laterally from andextending parallel to said vertical helix.
 7. The apparatus of claim 1,wherein said at least one support member comprises one or moretelescoping guides.
 8. The apparatus of claim 1, wherein there are apair of said guided lift mechanisms supported on said vehicle inlongitudinally spaced relation to one another.
 9. The apparatus of claim8, including a pair of bases detachably mounted on said vehicle in saidlongitudinally spaced relation and wherein said guided lift mechanismsare each mounted on a respective one of said bases so as to be readilymounted and dismounted from said vehicle.
 10. The apparatus of claim 9,wherein said bases are located at a predetermined height on said vehicleand wherein said vehicle includes a longitudinal region provided betweensaid bases.
 11. The apparatus of claim 10, wherein said longitudinalregion includes walk-through sections adjacent each of said bases havinga height about the same or less than that of said bases.
 12. Theapparatus of claim 11, wherein said walk-through sections have a heightabout the same as that of said bases.
 13. The apparatus of claim 8wherein said vehicle has a longitudinally extending interference freeregion, said lift mechanisms being located within said interference freeregion.
 14. The apparatus of claim 1, wherein said fixture is mounted onan upper base and wherein said push actuator said at least one supportmember are attached to a lower side of said upper base.
 15. Theapparatus of claim 14, wherein said fixture is attached to said upperbase via a floating plate mechanism that permits horizontal adjustmentof the position of said fixture relative to said upper base.
 16. Theapparatus of claim 1, wherein said at least one band comprises first andsecond continuous bands, said push actuator being operable to raise saidplaten by advancing said first and second bands upwardly in a helix withadjacent turns of said first band being spaced and individual turns ofsaid second band being inserted between the spaced turns of said firstband during upward advancement of said platen.
 17. A method ofsupporting and lifting an automotive chassis module for assembly with anautomotive body, said method comprising the steps of: loading thechassis module onto a fixture supported at a lowered position andcoupled to a push actuator mounted on a mobile vehicle; operating thepush actuator to raise the fixture vertically by upward advancement ofat least one helical band; and providing at least one support memberbetween the fixture and the vehicle to guide the fixture during itsvertical movement while restraining the fixture from horizontalmovement.
 18. The method of claim 17, including storing a retractedsection of the band in a coiled condition beneath the fixture.
 19. Themethod of claim 17, wherein said operating step further comprisesraising the fixture by upward advancement of a pair of interleavedhelical bands.
 20. The method of claim 19, including storing a retractedsection of a first one of said bands in a spiral coil beneath thefixture and storing a retracted section of the second of said bands in ahelical stack beneath the fixture.